1. Field of the Invention
This invention relates to a wavelength measuring device and a laser apparatus equipped with the wavelength measuring device for generating a laser of constant wavelength.
2. Description of the Related Art
FIG. 1 illustrates a conventional laser apparatus, for example, disclosed in the Japanese Patent Application Laid-Open No. 1-84681 (1989). In the drawing, references 1, 5 and 6 respectively denote a laser oscillator, a full reflection mirror and a partial reflection mirror disposed confronting the full reflection mirror 5 via the laser oscillator 1. A Fabry-Perot etalon (referred to as FP hereinafter) denoted by a reference number 7 is located between the laser oscillator 1 and the partial reflection mirror 6. The FP 7 is housed in a sealed container 8 filled with a gas. A volume increasing/decreasing means 9 by bellows is connected to the sealed container 8. The volume increasing/decreasing means 9 is driven by a servo mechanism 10. A laser beam 11 is oscillated by the laser oscillator 1, full reflecting mirror 5, partial reflecting mirror 6 and FP 7. A mirror 12 picks out a part of the laser beam 11, a sampling beam 13 to be measured. A wavelength measuring device 2 measures a wavelength of the sampling beam 13.
The wavelength measuring device 2 consists of an interference filter 14 allowing only the sampling beam 13 to pass through, a light intensity adjusting filter 15, an integrator 16 for dispersing the sampling beam 13, an air spaced FP 17 with a gap used as a monitor, a sealed container 18 in which the FP 17 is sealed, an image formation lens 19, a photosensor 20 of a one-dimensional image sensor to observe a fringe generated by the FP 17, an optical shielding box 21 for shutting off the outside light, a temperature regulator 22 for maintaining the FP 17 at a constant temperature, and a data processor 23 for analyzing the fringes observed by the photosensor 20. The output of the data processor 23 is inputted to the servo mechanism 10.
The operation will be depicted hereinafter. A wavelength of the laser beam 11 emitted from the laser oscillator 1 is selected by various kinds of elements in the oscillator 1. For example, the intrinsic width of an oscillating wavelength of an excimer laser is several angstroms, but the width is reduced when spectral elements such as a prism, a grating, an FP or the like are installed in the oscillator. Moreover, if the spectral elements are properly adjusted, an optional wavelength can be set within the width of the intrinsic oscillating wavelength. However, the selected wavelength is hard to be stabilized with high accuracy due to thermal deformation or vibration of the oscillator. As such, a wavelength of the laser is measured by introducing the sampling beam 13, a part of the laser beam 11, into the wavelength measuring device 2, and driving the servo mechanism 10 based on the measuring result to change the pressure of the ambience gas in the FP 7, thus stabilizing the wavelength of the laser.
The wavelength measuring device 2 uses the FP 17 for determining the wavelength. The FP 17 called as an air spaced Fabry-Perot etalon is obtained by bonding two highly flat mirrors 17a, 17b sandwiching a spacer 17c of thickness d. The center wavelength of the light passing through the mirrors 17a, 17b at an angle .theta. is represented by an equation (1) below: EQU .lambda.=2nd.times.cos.theta..sub.m /m (1)
wherein n is a refractive index of the gap, m is an integer indicating the degree and .theta..sub.m is an angle of the degree m.
When the laser beam has a divergence angle after passing through the integrator, only the beam component satisfying the above equation (1) among the beams entered the FP 17 penetrates the FP 17, forming a coaxial fringe (a ring-shaped interference fringe) centering the optical axis of the laser beam at the focal point of the image formation lens 19. As the photosensor 20 is arranged at the point focal of the image formation lens 19, the waveform having the light intensity distribution as shown in FIG. 2 is obtained. The abscissa in FIG. 2 indicates a distance x from the center of the fringe. The peak position x.sub.m of the fringe corresponding to the degree m is represented by the following equation (2): EQU x.sub.m =f.times..theta..sub.m ( 2)
Therefore, when the laser wavelength .lambda. changes, the intensity distribution changes from that indicated by a solid line to that by a broken line in FIG. 2. The change of the laser beam in wavelength can be operated from the change of the peak positions x.sub.m 's according to the equations (1) and (2).
As is understood from the equation (1), even when the wavelength .lambda. does not change, .theta..sub.m changes by a change in n or d, and accordingly the peak position x.sub.m of the fringes changes. In the conventional example, the FP 17 is sealed in the sealed container 18 and the density of the gas in the container 18 is kept constant so as to maintain the refractive index of the gap constant. Moreover, the temperature regulator keeps the temperature of the FP 17 constant so as to prevent the spacer 17c constituting the FP 17 from changing in thickness d of the spacer 17c as a result of thermal expansion.
When applying a KrF excimer laser as a light source of a lens reduction projection aligner in a semiconductor manufacturing apparatus, it is necessary to restrict the change of the laser wavelength not to exceed 0.5pm. Therefore, the allowable change .DELTA..lambda. in wavelength to satisfy the measuring accuracy required for the wavelength measuring device 2 is approximately 0.05pm. The spacer 17c of the FP 17 of the excimer laser is generally made of quartz glass having a thermal expansion coefficient a of 5.times.10.sup.-7. The change .DELTA..lambda. in wavelength when the temperature of the spacer 17 changes by .DELTA.T is expressed by an equation (3): EQU .DELTA..lambda.=.lambda..times..DELTA.d/d=.lambda..times..alpha..times..DEL TA.T (3)
Accordingly, the temperature change of the spacer 17c should not exceed 0.4.degree. C. so as to restrict the change in wavelength not to exceed 0.05pm. In consequence, the temperature regulator 22 should restrict the temperature change of the FP 17 not, to exceed 0.4.degree. C.
Since the conventional wavelength measuring device and the wavelength stabilizing laser apparatus equipped with the wavelength measuring device are constructed to always necessitate the use of temperature regulator 22 in order to execute considerably minute control of the temperature, the conventional device and apparatus are disadvantageously complicated and expensive.